Vocal communication among members of free ranging population of rats housed in a complex laboratory. Enclosures are either audible or are ultrasonic. Ultrasonic vocalizations tend to be more prolonged and to form patterned sequences. Rats vocalizing simultaneously are capable of utilizing separate frequency bands so that each vocalization signal can be distinguished. Rats required to perform cooperatively appear to exhibit different and, possibly more complex vocalization patterns, than those rats with free access to food and water resources.